Wettability-independent bouncing on flat surfaces mediated by thin air films

The impingement of drops onto solid surfaces1, 2 plays a crucial role in a variety of processes, including inkjet printing, fog harvesting, anti-icing, dropwise condensation and spray coating3, 4, 5, 6. Recent efforts in understanding and controlling drop impact behaviour focused on superhydrophobic surfaces with specific surface structures enabling drop bouncing with reduced contact time7, 8. Here, we report a different universal bouncing mechanism that occurs on both wetting and non-wetting flat surfaces for both high and low surface tension liquids. Using high-speed multiple-wavelength interferometry9, we show that this bouncing mechanism is based on the continuous presence of an air film for moderate drop impact velocities. This submicrometre ‘air cushion’ slows down the incoming drop and reverses its momentum. Viscous forces in the air film play a key role in this process: they provide transient stability of the air cushion against squeeze-out, mediate momentum transfer, and contribute a substantial part of the energy dissipation during bouncing

Single-enzyme analysis in a droplet-based micro- and nanofluidic system

The kinetic activity of individual enzyme molecules was determined in aqueous droplets generated in a nano- and microfluidic device. To avoid high background noise, the enzyme and substrate solution was confined into femtoliter carriers, achieving high product concentrations from single-molecule encapsulation. The tiny droplets (f ~ 2.5-3 µm) generated from this fluidic system were highly monodisperse, beneficial for an analysis of single enzyme activity. The method presented here allows to follow large numbers of individual droplets over time. The instrumental requirements are furthermore modest, since the small droplet size allows to use of standard microscope and standard Pyrex glass chips as well as the use of relatively high enzyme concentrations (nM range) for single molecule encapsulatio

Rapid prototyping of polymer-based rolled-up microfluidic devices

10.3390/mi9100516Micromachines91051

A highly efficient 3D micromixer fabricated by standard soft-lithography equipement

This paper reports a stereolithography-like 3D fabrication method based on soft-lithography techniques. It only requires standard equipment for photolithography, but it makes true 3D structures fabrication possible. We developed a rotating partition by this method in a microfluidic channel, which cannot be achieved by conventional soft-lithography, and demonstrated a prototyping three-dimensional flow mixer

Temperature-driven self-actuated microchamber sealing system for highly integrated microfluidic devices

We present here a novel microchamber sealing valve that is self-actuated by a pressure change during the temperature change in the thermal activation of reactions. Actuation of our valve requires only the use of the same heating device as employed for the reactions. A thermoplastic UV-curable polymer is used as a device material; the polymer allows realization of the temperature-driven valve actuation as well as the fabrication of multi-layered devices. The self-actuated valve achieves effective sealing of the microchamber for the polymerase chain reaction (PCR) even at 90 degrees C, which is essential for developing highly parallel PCR array devices without the need for complicated peripherals to control the valve operatio

Integrated biodetection in a nanofluidic device

The sensing of enzymatic processes in volumes at or below the scale of single cells is challenging but highly desirable in the study of biochemical processes. Here we demonstrate a nanofluidic device that combines an enzymatic recognition element and electrochemical signal transduction within a six-femtoliter volume. Our approach is based on localized immobilization of the enzyme tyrosinase in a microfabricated nanogap electrochemical transducer. The enzymatic reaction product quinone is localized in the confined space of a nanochannel in which efficient redox cycling also takes place. Thus, the sensor allows the sensitive detection of minute amounts of product molecules generated by the enzyme in real time. This method is ideally suited for the study of ultra-small-volume systems such as the contents of individual biological cells or organelles

Ultrarapid Generation of Femtoliter Microfluidic Droplets for Single-Molecule-Counting Immunoassays

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